Sensor device

ABSTRACT

An amperometric sensor device comprising a working electrode, a reference/pseudo-reference electrode and a permselective membrane incorporating a charged organic species. The electrodes are in intimate contact with the membrane and are on a side of the membrane opposite to the side exposed to a fluid sample. The charged organic species provides a conducting path through the membrane between the electrodes.

The present invention relates to sensor devices such as are used in thedetermination of a component or components which may be present in afluid sample such as a physiological fluid (e.g. blood, urine) or aneffluent.

PCT Application No. PCT/GB92/00443 (Publication No. WO-A-92/16647)discloses a sensor device which is suitable for detecting a targetedlipophilic component present in a fluid sample and providing an outputrepresentative of the content of the component in the sample. Thisdevice incorporates a permselective membrane which is substantiallyimpermeable to charged non-lipophilic species that may be present insamples to be analysed and being selective, by partitioning, forlipophilic species. As a result, a preselected lipophilic component inthe sample partitions into and diffuses through the barrier fordetection on the other side thereof.

In preferred embodiments of the aforesaid PCT application the membraneis of PVC incorporating a plasticiser (which acts as a permselectivemedium) such as dioctylphthalate or iso-propyl myristate. Such membranesare preferably used in an amperometric sensor incorporating anelectrochemical detection arrangement comprising an electrode system anda liquid or gel phase electrolyte medium which is interposed between themembrane and the electrodes. The electrode system may comprise a workingelectrode and a reference (or pseudo reference) electrode. Theelectrolyte medium provides a charge carrier system which ensures that acurrent may flow between the electrodes.

Whilst giving satisfactory results, the use of the electrolyte medium(which is usually, but not necessarily, aqueous) does however present anumber of disadvantages. In particular it complicates cell constructionand is not particularly suitable for use in dry reagent strip formatsensors.

It is therefore an object of the present invention to obviate ormitigate the abovementioned disadvantages.

According to the present invention there is provided a sensor device fordetecting a predetermined component present in a fluid sample andproviding an output representative thereof, the device comprising apermselective membrane incorporating a charged organic species, saidpermselective membrane being at least partially impermeable to chargedsubstantially non-lipophilic species that may be present in the sample,and an electrode arrangement in intimate contact with the membrane butnot exposed at the side thereof to be presented to the fluid sample.

Therefore in accordance with the invention the permselective membraneincorporates a charged organic species (which may function as a chargecarrier) and as such it is possible for the electrode arrangement to bein intimate contact with the membrane. The membrane therefore, combinesboth the functions of supporting electrolyte and permselective barrier.The need to provide a separate liquid or gel phase electrolyte mediumbetween the electrodes and the membrane is thus avoided. This providessignificant advantages including the possibility of highly simplifiedcell construction with no liquid reagents, such constructions beingsuitable in principle for dry reagent strip format sensors.

A further advantage associated with the use of the charged organicspecies is that organic phase electrochemistry can be exploited allowinga wider voltage range to be applied than can be used with aqueouselectrolyte layers (due to the break down of water at high voltages).

The electrode arrangement may be in intimate contact with the face ofthe membrane opposite to that face which is presented to the sample.Alternatively the necessary intimate contact may be achieved by virtueof the electrodes being incorporated within the membrane without beingexposed to the sample.

Sensor devices in accordance with the invention are preferablynon-potentiometric devices. Most preferably the sensor devices areamperometric devices in which the electrode system comprises a workingelectrode and a reference or pseudo-reference electrode system. Inoperation of such devices, the working electrode is maintained at apredetermined potential with respect to the reference electrode. Thispotential is selected to discharge a species of interest at the workingelectrode resulting an electric current which can be measured.

Examples of suitable charged organic species for use in the inventioninclude ion exchanges as used in ion selective electrodes (e.g. PVCbased ion selective electrodes).

The charged organic species may comprise a positive or negative organicion, associated in either case with an appropriate counter-ion (e.g. aninorganic ion). Alternatively the charged organic species may bezwitterionic. It is also to be understood that the term charged organicspecies covers organic species which, whilst not having a formal chargeper se, are closely associated with a charged species for example,organic molecules having a cage structure in which an ion isincorporated are covered by the term `charged organic species` as usedherein. The hydrophobic nature of the membrane ensures that the chargedorganic species is retained therein.

Examples of charged organic carrier species having a positive organicion are quaternary ammonium salts incorporating at least one, morepreferably at least 2, and most preferably at least 3 long chainsubstituted or unsubstituted hydrocarbon groups having 6 or more carbonatoms. Thus for example these groups may be aliphatic chains withoptional (e.g aromatic) substituents. Alternatively, there may be one ormore aryl groups bonded directly to the quaternary nitrogen atom. Thenegative counter ion is preferably a simple inorganic ion, e.g. halidesuch as chloride.

A preferred example of positive organic ion for use in the invention isthe tricaprylylmethyl ammonium ion.

Examples of charged organic species incorporating a negative organic ioninclude tetra-alkyl or tetra-aryl borates, e.g. a tetraphenyl borate.The counter ion may be a simple inorganic cation, e.g. an alkali metalcation. A preferred salt for use in the invention incorporating anegative organic ion is sodium tetraphenyl borate.

Further examples of charged organic species which may be used inaccordance with the invention include di-(n-long chain alkyl)phosphates, e.g. provided as the calcium salts. Particular examplesinclude calcium di-(n-decyl)phosphate, and calciumdi(n-(1,1,3,3-tetramethylbutyl))phosphate, and bis(2-ethylhexyl)hydrogenphosphate in which the hydrogen ion can be replaced by metal ions,particularly calcium. A further example is ferroinalkylbenzenesulphonate. A further possibility is to use ionophores as pendant groupswith chelated metal ions, or to use a redox active polymer orderivatives.

The charged organic species may be one which provides, or at leastcontributes to, the permselective properties of the membrane.

The presence of the charged species is likely to impart ion exchangeproperties. Hence the lipophilic selectivity of the membrane is likelyto be reduced as compared to membranes as disclosed in WO-A-92/16647.Furthermore membranes as disclosed herein might be suitable fortargeting non-lipophilic analytes, possibly charged.

The permselective membrane preferably comprises a synthetic polymermembrane incorporating the charged organic species. The polymer membraneis preferably plasticised, e.g. by virtue of the presence therein of thespecies used as the charged organic species or by additional plasticiser(e.g. dioctylphthalate).

The preferred polymer is PVC although other synthetic polymers may beused.

A membrane for use in the invention may be formulated by casting asolution (e.g. in THF) of the synthetic polymer (preferably PVC) andliquid tricapylylmethyl ammonium chloride (available under the nameAliquat 336), and allowing the solvent to evaporate. In this case thecharged organic species functions both as plasticiser and chargecarrier.

It is also possible for the charged organic species to be provided by asolid organic salt (e.g. sodium tetraphenyl borate). In this case, thesolid salt together with the polymer may be dissolved in an organicliquid (e.g. isopropyl myristate) which acts as plasticiser duringmembrane fabrication.

The amount of charged organic species incorporated in the membrane willdepend on the particular application. Additionally it should be notedthat excess organic species may allow significant interference frompermeating species facilitated by ion exchange properties of the organicspecies. In the case of a charged organic species which (with itscounterion) is a liquid then generally less than 10 μl (e.g. 2-5 μl) ofthe liquid will be used per 1 mg of polymer powder (e.g. PVC used forfabricating the membrane. If the charged organic species (with itscounterion) is a solid then generally a maximum of 35 mg of the solidwill be used per 60 mg of polymer together with 2-5 μl of plasticiserper 1 mg of polymer. The amounts quoted as to be understood asillustrative and not limiting.

The membrane may include additional components e.g. mediators, enzymes,cofactors, antibodies, reagents, and/or conducting particles, as well asincorporating a mixture of polymers (e.g. having ion exchangeproperties).

The membranes present in the sensor devices according to the inventionare at least partially impermeable to charged non-lipophilic species andare selective by partitioning for lipophilic species. As such, thedevice may be used for the determination of a lipophilic componentpresent in a sample. The lipophilic component of the sample may itselfbe directly detectable at the working electrode of the device. Ifhowever the component to be determined exhibits no or any insignificantelectrochemical activity the membrane may incorporate means forinteracting with the preselected component to produce a directlydetectable electrochemically active species.

Although the permselective membrane is selective towards lipophilicspecies, we do not exclude the possibility of the sensor may alsoexhibit some degree of selectivity towards a small uncharged hydrophilicspecies such a hydrogen peroxide and quinones. As such the sensor mayalso be used for the detection of such small molecules. Furthermore, asmentioned above, it is also possible that the membrane may be used forthe detection of charged species by virtue of the ion exchangeproperties of the charged organic species.

The types of analysis which may be effected with sensors in accordancewith the invention includes, but are not limited to, those disclosed inPCT/GB92/00443.

It will be appreciated that for any particular type of analysis to beeffected the composition of the membrane may be formulated to giveoptimum results. Thus, for example, if it is desired particularly toprevent negatively charged interferants passing across the membrane thenthe charged organic species may be a negative organic ion. This preventsthe possibility of ion exchange between the (relatively large) chargedorganic species and the interferant. Similarly the use of an positivelycharged organic ion may be used to prevent interference by positivelycharged ions.

The invention will be further described, by way of example only, withreference to the accompanying drawings, in which:

FIG. 1 schematically illustrates one embodiment of the sensor device inaccordance with the invention; and

FIGS. 2 and 3 illustrate (to an enlarged scale) various membranestructures.

FIG. 1 illustrates a sensor device 1 for use in amprometricmeasurements. The device comprises a permselective membrane 2 inintimate contact with electrodes 3 and 4 mounted in a support 5. At itsside opposite the support 5, the membrane 2 is in contact with bulk(analyte containing) solution 6 which may be aqueous or organic providedthat it does not dissolve the membrane. It will be appreciated that thesensor may be disposable and intended for one measurement only, in whichcase the requirement is that the solution does not dissolve the membranein the time scale of measurement.

Electrode 3 may be a working electrode (e.g. a platinum electrode) andelectrode 4 may be a reference/pseudo reference electrode (e.g. ofsilver).

The membrane 2 may for example comprise polyvinyl chloride incorporatinga charged organic species as described more fully above.

In use of the membrane, the polarisation of the electrodes andmeasurement of current in the cell is performed by appropriateamprometric methods. The provision of the organic charge carrier speciesin the membrane 2 provides for current flow between electrodes 3 and 4without the need for a separate liquid or gel electrolyte layer beinginterposed between the electrodes and the membrane.

The membrane may be of the form illustrated in FIG. 2 in which allmobile species are retained within the polymer matrix. Alternatively themembrane may be as shown in FIG. 3 in which mobile species are presentwithin the matrix of the polymer and also in a thin film 7 on themembrane surface which may result in the formation of a thin layerbetween membrane 2 and electrode 3 (and 4) during electrochemical cellconstruction.

Membranes for use in the sensor device may be produced by the followingprocedure which is given purely by way of example.

A membrane may be produced by dissolving PVC together with supportingelectrolyte in tetrahydrofuran. Thus, for example, 0.06 g of PVCtogether with an appropriate amount of supporting electrolyte (seebelow) may be dissolved in 10 ml of tetrahydrofuran. The solution ispoured into a 10 cm diameter petri dish, the lid replaced, and thetetrahydrofuran allowed to evaporate slowly at room temperature for 2-3days.

The organic charged carrier species may be provided by 150 μl Aliquat336 (tricaprylylmethyl ammonium chloride). The Aliquat 336 acts as botha charge carrier species and a plasticiser for the membrane.

Alternatively the charge carrier species may be provided by 35 mg ofsodium tetrophenyl borate used in conjunction with 150 μl of isopropylmyristate which acts as plasticiser during membrane fabrication.

It will be appreciated from the foregoing description that the inventionprovides a number of advantages as set out below.

The sensor incorporates a simple construction of a membrane coveredelectrode using a single combined membrane/electrolyte layer.

The membrane covered electrode may be pre-polarised and conditioned withno liquid present.

The construction of the sensor device does not involve aqueouscomponents. This avoids the need to lay down an aqueous layer which isan advantage if dry ingredients also need to be laid down.

The sensor device is able to exploit the advantages of organic phaseelectrochemistry e.g. wide applied voltage window, preconcentrationeffects and improvement in properties of any additional componentsrelated to the organic phase.

The membrane may be formulated to have a high degree of permselectivity(e.g. the use of a positively charged organic species will preventpositively charged interferants entering the membrane).

The absence of a separate liquid or gel phase electrolyte layer betweenthe membrane and the electrodes avoids problems with diffusion controlacross such a layer.

There is reduced electrode fouling with real samples.

The invention renders possible the exploitation of thin/fragilemembranes as intimate contact between the membrane and solid electrodeserves as support.

We claim:
 1. An amperometric sensor device for detecting a predeterminedcomponent present in a fluid sample and providing an outputrepresentative thereof, the device comprising a permselective membraneincorporating a charged organic species, said permselective membranebeing selective, by partitioning, for lipophilic species, and anelectrode arrangement having a working electrode and areference/pseudo-reference electrode, said electrodes of the electrodearrangement being in intimate contact with the membrane but not beingexposed at the side thereof to be presented to the fluid sample andwherein said charged organic species provides a conducting path throughthe membrane between said electrodes of the electrode arrangement. 2.The device as claimed in claim 1 wherein the membrane is of a syntheticpolymer.
 3. The device as claimed in claim 2 wherein the polymer is PVC.4. The device as claimed in claim 2 wherein the membrane is plasticized.5. The device as claimed in claim 4 wherein the plasticizer for themembrane is provided by the charged organic species.
 6. The device asclaimed in claim 1 wherein the charged organic species is provided bytricaprylylmethyl ammonium chloride.
 7. The device as claimed in claim 1wherein the charged organic species is provided by sodiumtetraphenylborate.
 8. The device as claimed in claim 1 wherein themembrane additionally incorporates at least one of a mediator, anenzyme, a cofactor, an antibody, a reagent, conducting particles, and amixture of polymers.
 9. The device as claimed in claim 1 wherein theelectrode arrangement is in intimate contact with a face of themembrane.
 10. The device as claimed in claim 1 wherein the electrodesare incorporated in the membrane.